Thomas Boston on Law and Gospel

Such is the natural propensity of man's heart to the way of the law, in opposition to Christ, that, as the tainted vessel turns the taste of the purest liquor put into it, so the natural man turns the very gospel into law, and transforms the covenant of grace into a covenant of works. - Thomas Boston, Human Nature in Its Fourfold State (Philadelphia: Presbyterian Board of Publications, 1860), page 70.